Hash passwords with 1 MiB in tests so make test fits in 4 GiB (closes #261)

On a build machine with 4 GiB, docker build . failed: the auth test
binary was killed for lack of memory. upaasd hashes passwords with
argon2id at 64 MiB per hash; the auth and handlers tests run many such
hashes at once, and the race detector multiplies what each one takes,
so make test peaked at 2.7 GiB.

The memory per hash is now the auth service's ArgonMemory field. New
sets it to the same 64 MiB, which upaasd keeps; the test helpers lower
it to 1 MiB. A new test checks that New sets 64 MiB and hashes and
verifies a password with it. GOMAXPROCS=4 make test now peaks at about
1.4 GiB.

Model: opus-5-5

TODO.md

@@ -20,6 +20,12 @@ regress.
 
 # Completed Steps
 
+- 2026-10-03: `make test`, and so `docker build .`, fits a machine with 4 GiB of
+  memory: tests hash passwords with 1 MiB instead of upaasd's 64 MiB, so
+  `GOMAXPROCS=4 make test` peaks at about 1.4 GiB instead of 2.7 GiB. upaasd
+  still hashes with 64 MiB, and one test hashes and verifies a password at that
+  cost (#261).
+
 - 2026-10-02: In a window too narrow for the top bar, such as 390 px, the New
   App and Logout buttons move to a second row instead of running into "by
   @sneak"; the bar keeps a gap between its two sides at every width (#272).